Network Working Group E. Wilde
Internet-Draft EMC
Updates: 5261 (if approved) June 18, 2013
Intended status: Informational
Expires: December 20, 2013
A Media Type for XML Patch Operations
draft-wilde-xml-patch-05
Abstract
The XML Patch media type "application/xml-patch+xml" defines an XML
document structure for expressing a sequence of patch operations that
are applied to an XML document. The XML Patch document format's
foundations are defined in RFC 5261, this specification defines a
document format and a media type registration, so that XML Patch
documents can be labeled with a media type, for example in HTTP
conversations.
In addition to the media type registration, this specification also
updates RFC 5261 in some aspects, limiting these updates to cases
where RFC 5261 needed to be fixed, or was hard to understand.
Note to Readers
This draft should be discussed on the apps-discuss mailing list [14].
Online access to all versions and files is available on github [15].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 20, 2013.
Copyright Notice
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Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Patch Document Format . . . . . . . . . . . . . . . . . . . . 3
3. Patch Examples . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Implementation Hints . . . . . . . . . . . . . . . . . . . . . 6
6.1. Matching Namespaces . . . . . . . . . . . . . . . . . . . 7
6.2. Patching Namespaces . . . . . . . . . . . . . . . . . . . 8
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. From -04 to -05 . . . . . . . . . . . . . . . . . . . . . 10
8.2. From -03 to -04 . . . . . . . . . . . . . . . . . . . . . 10
8.3. From -02 to -03 . . . . . . . . . . . . . . . . . . . . . 10
8.4. From -01 to -02 . . . . . . . . . . . . . . . . . . . . . 10
8.5. From -00 to -01 . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Non-Normative References . . . . . . . . . . . . . . . . . 11
Appendix A. Updates to RFC 5261 . . . . . . . . . . . . . . . . . 12
A.1. Section 4.2.2 . . . . . . . . . . . . . . . . . . . . . . 12
A.2. Section 4.4.3 . . . . . . . . . . . . . . . . . . . . . . 13
A.3. Section 8 . . . . . . . . . . . . . . . . . . . . . . . . 14
A.4. XSD for RFC 5261 . . . . . . . . . . . . . . . . . . . . . 14
A.5. ABNF for RFC 5261 . . . . . . . . . . . . . . . . . . . . 16
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
The Extensible Markup Language (XML) [1] is a common format for the
exchange and storage of structured data. HTTP PATCH [6] extends HTTP
[7] with a method to perform partial modifications to resources.
HTTP PATCH requires that patch documents are being sent along with
the request, and it is therefore useful if there are standardized
patch document formats (identified by media types) for popular media
types.
The XML Patch media type "application/xml-patch+xml" is an XML
document structure for expressing a sequence of operations to apply
to a target XML document, suitable for use with the HTTP PATCH
method. Servers can freely choose which patch formats they want to
accept, and "application/xml-patch+xml" could be a simple default
format that can be used unless a server decides to use a different
(maybe more sophisticated) patch format for XML.
The format for patch documents is based on the XML Patch Framework
defined in RFC 5261 [2]. While RFC 5261 does define a concrete
syntax as well as the media type "application/patch-ops-error+xml"
for error documents, it only defines XML Schema (XSD) [8] types for
patch operations, and thus the concrete document format and the media
type for patch operations are defined in an XSD defined in this
specification.
Since RFC 5261 contains sections that need to be fixed, or are hard
to understand, this specification updates RFC 5261. The updates are
listed in Appendix A, and all references to RFC 5261 made in this
specification should be read as referring to the updated version.
The main reason for the changes are the problematic ways in which RFC
5261 relies on XPath as the expression language for selecting the
location of a patch, while at the same time XPath's data model does
not contain sufficient information to determine whether such a
selector indeed can be used for a patch operation, or should result
in an error. Specifically, the problem occurs with namespaces, where
XPath does not expose namespace decalration attributes, while the
patch model needs them to determine whether a namespace patch is
allowed or not. Section 6 contains more information about the
general problem, and Appendix A lists the resulting updates to RFC
5261 to make the model well-defined and the text easier to read and
understand.
2. Patch Document Format
The XML patch document format is based on a simple schema that uses a
"patch" element as the document element, and allows an arbitrary
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sequence of "add", "remove", and "replace" elements as the children
of the document element. These children follow the semantics defined
in RFC 5261, which means that each element is treated as an
individual patch operation, and the result of each patch operation is
a patched XML document that is the target XML document for the next
patch operation.
The following example patch document uses the example from RFC 5261,
and simply uses a "patch" element and a new XML namespace. It shows
the general structure of an XML patch document, as well as an example
for each operation.
Patched doc
new attr
As this example demonstrates, both the document element "patch" and
the patch operation elements are in the same XML namespace. This is
the result of RFC 5261 only defining types for the patch operation
elements, which then can be reused in schemas to define concrete
patch elements.
RFC 5261 defines an XML Schema (XSD) [8] for the patch operation
types, which is shown in Appendix A.4. The following schema for the
XML Patch media type is based on the types defined in RFC 5261, which
are imported as "rfc5261.xsd" in the following schema. The schema
defines a "patch" document element, and then allows an unlimited (and
possibly empty) sequence of the "add", "remove", and "replace"
operation elements, which are directly based on the respective types
from the schema defined in RFC 5261.
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3. Patch Examples
Since the semantics of the XML patch operations are defined by RFC
5261, please refer to the numerous examples in that specification for
concrete XML patch document examples. Most importantly, the examples
in RFC 5261 can be taken literally as examples for the XML Patch
media type, as long as it is assumed that the XML namespace for the
operation elements in these examples is the URI "urn:ietf:rfc:XXXX".
4. IANA Considerations
The Internet media type [3] for an XML Patch Document is application/
xml-patch+xml.
Type name: application
Subtype name: xml-patch+xml
Required parameters: none
Optional parameters: Same as charset parameter for the media type
"application/xml" as specified in RFC 3023 [1].
Encoding considerations: Same as encoding considerations of media
type "application/xml" as specified in RFC 3023 [1].
Security considerations: This media type has all of the security
considerations described in RFC 3023 [1] and RFC 5261 [2], plus
those listed in Section 5.
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Interoperability considerations: N/A
Published specification: RFC XXXX
Applications that use this media type: Applications that
manipulate XML documents.
Additional information:
Magic number(s): N/A
File extension(s): XML documents should use ".xml" as the file
extension.
Macintosh file type code(s): TEXT
Person & email address to contact for further information: Erik
Wilde
Intended usage: COMMON
Restrictions on usage: none
Author: Erik Wilde
Change controller: IETF
5. Security Considerations
Parsing XML may entail including information from external sources
through XML's mechanism of external entities. Implementations
therefore should be aware of the fact that standard parsers may
resolve external entities, and thus include external information as a
result of applying patch operations to an XML document.
6. Implementation Hints
This section is informative. It described some issues that might be
interesting for implementers, but it might also be interesting for
users of XML Patch that want to understand some of the differences
between standard XPath 1.0 processing, and the processing model of
selectors in RFC 5261.
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6.1. Matching Namespaces
RFC 5261 defines standard rules for matching prefixed names in
expressions: Any prefixes are interpreted according to the namespace
bindings of the diff document (the document that the expression is
applied against). This means that each prefixed name can be
interpreted in the context of the diff document.
For unprefixed names in expressions, the rules depart from XPath 1.0
[9]. XPath 1.0 defines that unprefixed names in expressions match
namespace-less names (i.e., there is no "default namespace" for names
used in XPath 1.0 expressions). RFC 5261 requires, however, that
unprefixed names in expressions must use the default namespace of the
diff document (if there is one). This means that it is not possible
to simply take a selector from a patch document and evaluate it in
the context of the diff document according to the rules of XPath 1.0,
because this would interpret unprefixed names incorrectly. As a
consequence, it is not possible to simply take an XPath 1.0 processor
and evaluate XMPL Patch selectors in the context of the diff
document.
As an extension of XPath 1.0's simple model, XPath 2.0 [10] specifies
different processing rules for unprefixed names: They are matched
against the URI of the "default element/type namespace", which is
defined as part of an expression's static context. In some XPath 2.0
applications, this can be set; XSLT 2.0 for example has the ability
to define an "xpath-default-namespace", which then will be used to
match unprefixed names in expressions. Thus, by using an XPath 2.0
implementation that allows to set this URI, and setting it to the
default namespace of the diff document (or leaving it undefined if
there is no such default namespace), it is possible to use an out-of-
the-box XPath 2.0 implementation for evaluating XML Patch selectors.
Please keep in mind, however, that evaluating selectors is only one
part of applying patches. When it comes to applying the actual patch
operation, neither XPath 1.0 nor XPath 2.0 are sufficient, because
they are not preserving some of the information from the XML syntax
(specifically: namespace declarations) that is required to correctly
apply patch operations. The following section described this issue
in more detail.
Please note that RFC 5261's Section 4.2.2 on namespace matching
explains XPath 2.0's rules incorrectly
. For this reason,
Appendix A.1 updates Section 4.2.2 of RFC 5261.
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6.2. Patching Namespaces
One of the issues when patching namespaces based on XPath is that
XPath exposes namespaces different than the XML 1.0 [11] syntax for
XML Namespaces [12]. In the XML syntax, a namespace is declared with
an attribute using the reserved name or prefix "xmlns", and this
results in this namespace being available recursively through the
document tree. In XPath, the namespace declaration is not exposed as
an attribute (i.e., the attribute, although syntactically an XML
attribute, is not accessible in XPath), but the resulting namespace
nodes are exposed recursively through the tree.
RFC 5261 uses the terms "namespace declaration" and "namespace"
almost interchangeably, but it is important to keep in mind that the
namespace declaration is an XML syntax construct that is unavailable
in XPath, while the namespace itself is a logical construct that is
not visible in the XML syntax, but a result of a namespace
declaration. The intent of RFC 5261 is to patch namespaces as if
namespace declarations were patched, and thus it only allows to patch
namespace nodes on the element nodes where the namespace has been
declared.
Patching namespaces in XML Patch is supposed to "emulate" the effect
of actually changing the namespace declaration (which is why a
namespace can only be patched at the element where it has been
declared). Therefore, when patching a namespace, even though XPath's
"namespace" axis is used, implementations have to make sure that not
only the single selected namespace node is being patched, but that
all namespaces nodes resulting from the namespace declaration of this
namespace are also patched accordingly.
This means that an implementation might have to descend into the
tree, matching all namespace nodes with the selected prefix/URI pair
recursively, until it encounters leaf elements or namespace
declarations with the same prefix it is patching. Determining this
requires access to the diff document beyond XPath, because in XPath
itself namespace declarations are not represented, and thus such a
recursive algorithm wouldn't know when to stop. Consider the
following document:
If this document is patched with a selector of /x/namespace::a, then
only the namespace node on element x should be patched, even though
the namespace node on element y has the same prefix/URI combination
than the one on element x. However, determining that the repeated
namespace declaration was present at all on element y is impossible
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when using XPath alone, which means that implementations must have an
alternative way to determine the difference between the document
above, and this one:
In this second example, patching with a selector of /x/namespace::a
should indeed change the namespace nodes on elements x and y, because
they both have been derived from the same namespace declaration.
The conclusion of these considerations is that for implementing XML
Patch, access closer to the XML syntax (specifically: access to
namespace declarations) is necessary. As a result, implementations
attempting to exclusively use the XPath model for implementing XML
Patch will fail to correctly address certain edge cases (such as the
one shown above).
Note that XPath's specific limitations do not mean that it is
impossible to use XML technologies other than XPath. The Document
Object Model (DOM) [13], for example, does expose namespace
declaration attributes as regular attributes in the document tree,
and thus could be used to differentiate between the two variants
shown above.
Please note that RFC 5261's Section 4.4.3 on replacing namespaces
mixes the terms "namespace declaration" and "namespace". For this
reason, Appendix A.2 updates Section 4.4.3 of RFC 5261.
7. Implementation Status
Note to RFC Editor: Please remove this section before publication.
As explained in a draft currently under development
, this section
contains information about implementation status, so that reviews of
the draft document can take implementation reports into account as
well. If you are implementing this draft, please contact this
draft's author. Any implementation status reports are intended for
draft publications only; the section will be removed when the draft
is published in RFC form.
EMC: We have implemented the selector part of the specification,
which is the trickiest part (see Section 6.1 for an explanation).
By reusing an existing XPath 1.0 implementation and changing it to
match the changed default namespace processing model, the required
behavior is fairly easy to implement. This does, however, require
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that the implementation is available in source code, and also does
require some changes to the implementation's code. The resulting
implementation is closed source and will be made available, if
released, as part of EMC's XML database product xDB
.
8. Change Log
Note to RFC Editor: Please remove this section before publication.
8.1. From -04 to -05
o Improved formatting of XML/XSD and ABNF code.
o Moving category from "std" to "info" (intended to become an
informational RFC).
8.2. From -03 to -04
o Added text and section Appendix A about updating RFC 5261 (instead
of relying on errata).
8.3. From -02 to -03
o Added section on "Implementation Status" (Section 7).
o Improved "Implementation Hints" (Section 6).
8.4. From -01 to -02
o Textual edits.
o Added section on "Implementation Hints" (Section 6).
8.5. From -00 to -01
o Removed Mark Nottingham from author list.
o Changed media type name to application/xml-patch+xml (added suffix
per draft-ietf-appsawg-media-type-suffix-regs)
o Added ABNF grammar derived from XSD (Appendix A.5)
9. References
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9.1. Normative References
[1] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
RFC 3023, January 2001.
[2] Urpalainen, J., "An Extensible Markup Language (XML) Patch
Operations Framework Utilizing XML Path Language (XPath)
Selectors", RFC 5261, September 2008.
[3] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC 6838,
January 2013.
[4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[5] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
9.2. Non-Normative References
[6] Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789,
March 2010.
[7] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[8] Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn, "XML
Schema Part 1: Structures Second Edition", World Wide Web
Consortium Recommendation REC-xmlschema-1-20041028,
October 2004,
.
[9] DeRose, S. and J. Clark, "XML Path Language (XPath) Version
1.0", World Wide Web Consortium Recommendation REC-xpath-
19991116, November 1999,
.
[10] Boag, S., Berglund, A., Kay, M., Simeon, J., Robie, J.,
Chamberlin, D., and M. Fernandez, "XML Path Language (XPath)
2.0 (Second Edition)", World Wide Web Consortium
Recommendation REC-xpath20-20101214, December 2010,
.
[11] Sperberg-McQueen, C., Yergeau, F., Paoli, J., Maler, E., and T.
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Bray, "Extensible Markup Language (XML) 1.0 (Fifth Edition)",
World Wide Web Consortium Recommendation REC-xml-20081126,
November 2008, .
[12] Hollander, D., Layman, A., Bray, T., Tobin, R., and H.
Thompson, "Namespaces in XML 1.0 (Third Edition)", World Wide
Web Consortium Recommendation REC-xml-names-20091208,
December 2009,
.
[13] Robie, J., Wood, L., Champion, M., Hegaret, P., Nicol, G., Le
Hors, A., and S. Byrne, "Document Object Model (DOM) Level 3
Core Specification", World Wide Web Consortium
Recommendation REC-DOM-Level-3-Core-20040407, April 2004,
.
URIs
[14]
[15]
Appendix A. Updates to RFC 5261
This section is normative. It contains a list of updates to RFC 5261
[2]. These updates are limited to cases where RFC 5261 needed to be
fixed, or was hard to understand.
A.1. Section 4.2.2
Section 4.2.2 of RFC 5261 [2] says:
In XPath 2.0, a "bar" selector not only matches an unqualified
element, but also matches a qualified element that is
in scope of a default namespace declaration. In contrast, in this
specification, a selector without a prefix only matches one
element, and it may match an element with or without a prefix but
only if the namespace it's qualified with (or none) is an exact
match.
It should say:
In XPath 2.0, a "bar" selector matches elements that have the URI
of the "default element/type namespace", which is part of an
XPath's static context. By setting this URI to the default
namespace of the diff document (or leave it empty, if there is
none), XPath 2.0's behavior matches the requirements of the
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previous section.
Explanation: The original text is not easy to understand, but seems
to assume that an unprefixed name in XPath 2.0 matches both
unprefixed names, and prefixed ones that have the same namespace as
the default namespace of the XPath static context. This is not the
case: Matching depends on how the "default element/type namespace" of
the XPath static context is defined, and then matches either
namespace-less elements, or those in the "default element/type
namespace", but never both. This context, however, is defined by the
XPath itself, not by the document. Thus, it can be set externally
and could be set to the diff document's default namespace (if there
is one). In that case, XPath 2.0 can be used to evaluate XML Patch
selectors.
A.2. Section 4.4.3
Section 4.4.3 of RFC 5261 [2] says:
4.4.3. Replacing a Namespace Declaration URI
An example for a replacement of a namespace URI: urn:new:xxx This will replace
the URI value of 'pref' prefixed namespace node with
"urn:new:xxx". The parent node of the namespace declaration MUST
be the element, otherwise an error occurs.
It should say:
4.4.3. Replacing a Namespace URI
An example for a replacement of a namespace URI: urn:new:xxx This will replace
the URI of the namespace associated with the 'pref' prefix with
"urn:new:xxx". The parent node of the namespace declaration MUST
be the element, otherwise an error occurs. Replacing the
namespace at the element where it is declared MUST also change all
namespace nodes derived from this declaration in descendant
elements.
Explanation: The specification uses the terms "namespace declaration"
and "namespace" almost interchangeably, which is incorrect. It is
impossible to select namespace declarations using XPath. When
selecting and replacing a namespace, then it should be taken into
account that its associated namespace declaration very likely has
resulted in numerous namespace nodes, attached to child elements of
the element where the namespace was declared. It is likely that RFC
5261 intended to specify a "recursive replace" of the resulting
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namespace nodes of a namespace declaration, and this is what the
corrected text suggests. The original text is mixing terminology,
hard to read, and ambiguous in its meaning.
Side note: If the original text indeed tried to specify that really
only this one namespace node should be changed, then this could lead
to rather strange effects in the resulting document, since the XPath
tree now would have "orphan" namespace nodes, which then would need
to be serialized, and there would be resulting namespace declarations
in locations where previously no namespace declarations occurred.
A.3. Section 8
Section 8 of RFC 5261 [2] says:
It should say:
Explanation: The regex in the XSD suggests that "id()" would be a
valid selector for a patch, but it would not make sense to specify
such a selector, since it never would select a node (there's no
identifier to locate in the document). This means that while "id()"
is a valid XPath expression, it should not be allowed as a selector
expression within an XML patch document.
A.4. XSD for RFC 5261
This section contains a modified copy of the XML Schema (XSD) [8]
defining the add, replace, and remove types in RFC 5261 [2]. The
modification is based on the grammar change made in Appendix A.3.
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]>
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A.5. ABNF for RFC 5261
RFC 5261 [2] does not contain an ABNF grammar for the allowed subset
of XPath expressions, but includes an XSD-based grammar in its type
definition for operation types (which is shown in Appendix A.4). In
order to make implementation easier, this appendix contains an ABNF
grammar that has been derived from the XSD expressions given in
Appendix A.4. In the following grammar, "xpath" is the definition
for the allowed XPath expressions for remove and replace operations,
and "xpath-add" is the definition for the allowed XPath expressions
for add operations. The names of all grammar productions are the
ones used in the XSD-based grammar of RFC 5261.
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ncname = 1*%x00-ffffffff
qname = [ ncname ":" ] ncname
aname = "@" qname
pos = "[" 1*DIGIT "]"
attr = ( "[" aname "='" 1*%x00-ffffffff "']" ) /
( "[" aname "=" DQUOTE 1*%x00-ffffffff DQUOTE "]" )
valueq = "[" ( qname / "." ) "=" DQUOTE 1*%x00-ffffffff DQUOTE "]"
value = ( "[" ( qname / "." ) "='" 1*%x00-ffffffff "']" ) / valueq
cond = attr / value / pos
step = ( qname / "*" ) 0*( cond )
piq = "processing-instruction(" [ DQUOTE ncname DQUOTE ] ")"
pi = ( "processing-instruction(" [ "'" ncname "'" ] ")" ) / piq
id = ( "id(" [ "'" ncname "'" ] ")" ) /
( "id(" [ DQUOTE ncname DQUOTE ] ")" )
com = "comment()"
text = "text()"
nspa = "namespace::" ncname
cnodes = ( text / com / pi ) [ pos ]
child = cnodes / step
last = child / aname / nspa
xpath = [ "/" ] ( ( id [ 0*( "/" step ) "/" last ] ) /
( 0*( step "/" ) last ) )
xpath-add = [ "/" ] ( ( id [ 0*( "/" step ) "/" child ] ) /
( 0*( step "/" ) child ) )
Appendix B. Acknowledgements
Thanks for comments and suggestions provided by Bas de Bakker.
Author's Address
Erik Wilde
EMC
6801 Koll Center Parkway
Pleasanton, CA 94566
U.S.A.
Phone: +1-925-6006244
Email: erik.wilde@emc.com
URI: http://dret.net/netdret/
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